1. Field of the Invention
The present invention relates to a method of and an apparatus for correcting the color of a print medium and a proofer such as a color printer or the like, in a proof generating system which generates a color proof comprising a color image printed on a proof sheet with the proofer, before a colored printed material is produced using a print sheet by a color printing machine such as a rotary press or the like.
2. Description of the Related Art
It has heretofore been customary to generate a color proof for examining colors and making color corrections with a proofer such as a color printer or the like before a colored printed material with a color image in the form of a halftone-dot image printed on a print sheet as a final product is produced by a color printing machine.
The proofer is used because it does not require films and printing plates (presensitized plates) to be produced and can easily generate a plurality of hard copies or color proofs within a short period of time.
For forming a color image on a proof sheet, image data depending on a device such as a printing machine, a camera, an image sensor, a CRT, an LED, etc., e.g., C, M, Y, K (cyan, magenta, yellow, black) image data, are converted to calorimetric data independent of a device, e.g., X, Y, Z (stimulus value) data, according to a standard printing profile (a four-dimensional lookup table or the like) given by a printer manufacturer or the like. Then, the device-independent calorimetric data are converted to device-dependent image data for a color printer, e.g., R, G, B (red, green, blue) image data, according to a proofer profile, e.g., a printer profile (a three-dimensional lookup table). Using the device-dependent image data, a color proof with a color image formed on a proof sheet is generated by a color printer which is also referred to as a proof printer. In this manner, the colors of a printed material to be produced by the printing machine can be simulated by the color proof thus generated before the printed material is actually produced.
However, it often happens for the standard printing profile to fail to match printing properties, i.e., printing conditions depending on inks, papers, and printing machine characteristics, of the printing machine which will actually be used to print the color image.
Specifically, the printing conditions vary depending on many parameters which include not only inks, papers, printing machine types, but also production lots and ambient temperatures even when the same inks, paper, and printing machine are used. It is impossible for the standard printing profile to fully match actual printing conditions used by the user, i.e., desired printing conditions. It is therefore necessary to adjust the printing profile according to actual printing conditions, i.e., desired printing conditions, rather than standard printing conditions.
Inks and papers that are in general use are limited to certain types. Therefore, solid ink colors and paper colors are relatively stable even under different printing conditions. However, intermediate color tones may vary greatly depending on conditions in which color images are printed by an actual printing machine. For adjusting such intermediate color tones, it has been the practice to insert one-dimensional lookup tables for the respective colors C, M, Y, K to adjust C, M, Y, K tone curves (also referred to as gradation characteristics or dot gains) prior to the four-dimensional lookup table as the standard printing profile, and convert the respective colors C, M, Y, K to C′, M′, Y′, K′, respectively, for color correction.
If the proofer is a CMYK printer capable of using print sheets for finally printing color images thereon, then when such print sheets of the same type and lot as will be used by the actual printing machine are used by the proofer, color images printed on print proofs by the proofer can be confirmed as reflecting finally printed color images substantially exactly.
However, if sheets used by the proofer are of the type for exclusive use by the proofer and are different from print sheets according to the standard printing profile for use by the actual printing machine, then adjustments need to be made because the color of the print sheets themselves does not match the gradation (dot gain) of highlights (lightly tinted areas). Such adjustments can be made to a certain extent by adjusting the tone curves with the one-dimensional lookup tables. However, the sheet color can only be adjusted in principle by increasing the amount of inks. If the sheet color is adjusted, then not only the sheet color, but also the dot gain of lightly tinted areas or highlights is varied, and it is not clear how the dot gain of lightly tinted areas or highlights is to be adjusted.
According to the conventional practice in which proofs are produced by the proofer using dedicated sheets, if the color of print sheets to be used by the actual printing machine is lighter than the color of print sheets according to the standard printing profile, then it is impossible to adjust the dot gain with the sheet color.